The present invention relates to a subsea well installation and, more particularly, to a multiline riser support and connection system which operates in engagement with or detachment from a floating platform.
Conventionally, in subsea well drilling and production operations, a floating platform or vessel on the surface of the sea is connected to a subsea well head by a riser run. The riser run may be either a drilling or production run and is comprised of a plurality of riser sections assembled by supporting the uppermost portion of a riser section and subsequently adding riser sections and lowering the assembled riser sections until the desired overall length is reached.
In such operations, a spider may be used which is disposed above a rotary table mounted on the vessel. The spider is opened to encircle the riser thus permitting it to be lowered through the spider. The spider is then closed to support the riser section upon the table by means of a flange on the uppermost end of each riser section. Each new riser section is added to the uppermost section of the spider supported riser run by means of a derrick, or the like, mounted on the vessel which lifts a new riser section over the assembled riser run and lowers it upon the top of the riser run where the flange of the newly positioned riser is bolted or otherwise secured to the flange of the uppermost portion of the assembled riser. The weight of the riser run is then lifted by the derrick, the spider is opened, and the assembled risers or lowered into the sea to repeat the operation.
During such operations, it is desirable to maintain a constant load on the riser run regardless of the motion of the vessel or platform. An example of a hydraulically operated gimbal system which supports the table that, in turn, supports the run is shown in U.S. Pat. No. 3,984,900, issued Oct. 12, 1976.
As each riser section is lowered into the sea, its weight is supported to a substantial extent by gas entrapped between the outer surface of the cylindrical wall which forms the inner tube of the riser and the inner surface of a cylindrical shell which forms the outer cover of the riser. This entrapped gas contributes to the buoyancy of the riser as described in greater detail in U.S. Pat. No. 3,858,401, issued Jan. 7, 1975. The inner tube formed by the cylindrical wall may be used for a clearance passageway for drilling tools attached to an operating string when the riser run is used for drilling operations or as a passageway through which the production of the well is brought to the surface during the production operation.
Once the riser run has been landed on the well head and connection is made with the well head connector, as shown in U.S. Pat. No. 4,109,712, issued Aug. 29, 1978, the vertical motion of the floating platform or vessel must be absorbed to prevent the riser run from crushing under its own weight. An example of a telescoping joint used on buoyant riser sections to absorb the motion of a vessel is shown in U.S. Pat. No. 3,952,526, issued Apr. 27, 1976.
The telescoping joint which compensates for the vertical movement of the floating platform may be located at either the lower end of the riser string adjacent the well head or the upper end of the riser string adjacent the platform. When the telescoping joint is located at the upper end of the riser run adjacent the vessel, the run is dynamically hung from cables attached from the lower surface of the platform or vessel to a point below the telescoping joint. Winches are provided on the cables to retain a constant tension and prevent the riser run from buckling under its own weight. An example of such a tensioning system is shown in U.S. Pat. No. 3,791,442, issued Feb. 12, 1974.
In all of the systems described above, the multilines or hoses connected between the floating platform or vessel and the riser run which are ultimately connected to the well head must pass from the vessel to the uppermost riser and connect thereto by threaded connectors or other suitable means. These lines include flow lines, injection lines, and sales lines when the drill rig is being operated in the production phase. In the drilling operation, the lines include supply and return lines for a hydraulic drilling fluid, commonly called mud, and choke and kill lines for blowout protection. An example of a blowout-preventer used in an upper most riser of a drilling riser run is shown in the U.S. Pat. No. 3,791,442 patent.
While a floating platform generally operates in quiescent seas, clearly the platform or vessel will be subjected to storms during some of its drilling or production operation. At this time, the operation is terminated and the riser run is withdrawn from the well head. The run is then attached to the floating platform or vessel and allowed to extend vertically into the sea or "hung off" until the storm has passed. During a conventional emergency disconnect, the system described above requires men to disconect the multilines or hoses from the riser run while hung off in a bosun's chair in a dangerous area called the "moonpool" area. The moonpool is the well exposed to the sea which surrounds the riser run and extends through the hull of the ship.